The present invention relates to treads of tires having a carcass reinforcement surmounted by a crown reinforcement belt.
The problem of reducing the energy consumption of a vehicle is an important one and frequently is controlling in the development and sale of certain tires. The solution to this problem requires the production of tires which have a reduced resistance to rolling.
It is known that the resistance to rolling of a tire on a road surface is related to losses of energy in said tire which are in part dependent on the hysteresis characteristics of the rubber mixes employed, particularly those forming the tread and in part dependent on the deformation cycles experienced by said mixes during the rolling of the tire.
One possibility for reducing losses of energy of a rolling tire concerns reduction of the energy losses resulting from the hysteresis characteristics of the rubber mixtures employed, and particularly of the mixtures constituting the tread.
Another possibility for the reduction of the energy losses concerns the manner in which the tread is provided with cuts in the form of notches and/or incisions, in order to form a tread pattern.
In fact, if one examines the tread of a tire inflated to a pressure corresponding to its pressure of use and supporting a given load it is found that said tread has a surface, known as the rolling surface, located radially to the outside of the tire, which, outside of the zone of contact with the ground, can be characterized as a surface of double radius of curvature, namely curved both in the meridian direction and in the circumferential direction. This surface is characterized by the fact that its intersection with a radial plane, that is to say a plane which contains the axis of rotation of the tire, has a profile which at all points has a meridian radius of curvature other than zero; similarly, and obviously, the intersections of the rolling surface with planes perpendicular to the axis of rotation of the tire correspond to circles the radii of curvature of which are almost identical to the radius of the inflated tire not under load, measured in the equatorial plane. The equatorial plane is the plane perpendicular to the axis of rotation of the tire, passing through the center of the axial width of the tread.
The term "notch", as used herein means any cut produced in the tread and creating two walls facing each other and extending to the tread surface and having a width of at least 2 mm, that is to say the average distance separating the walls of the notch between the mouth and the bottom of the notch is, on the average, 2 mm; by "incision", there is understood a cut of a width of less than 2 mm.
Upon rolling on ground of constant average curvature along a main direction corresponding to the direction of rolling, the rolling surface is deformed in a region called the contact region so as to adopt the geometry of the ground and therefore the principal curvatures of the ground; this contacting therefore results in deformations of the tread. Upon the entry into and departure from the zone of contact with the ground, the tread and its rolling surface pass through two transition zones, located in the vicinity of the contact zone, in which said tread undergoes strong circumferential flexure and is forced to assume a very small radius of curvature in the circumferential direction.
Accordingly, the circumferential flexure deformation cycle undergone by the tread is very important since it impels the outer surface of the tread to pass through very large curvature changes upon entrance into and departure from contact, which surface must assume the average curvature of the ground between these two zones.
By average curvature of the ground, it is understood that the ground on which the tire rolls has a single curvature in the direction of displacement of the tire thereon and that this curvature remains close to a fixed value; a curvature of zero corresponds to a flat ground while to a rolling wheel for the testing of a tire there corresponds a curvature the value of which may be positive or negative depending on whether the ground is concave or convex.
It has been proposed, for instance in French Patent 1 080 845, in order to reduce the amount of energy dissipated by the deformation of the tread upon entrance into and departure from contact, to provide said tread with numerous incisions and/or grooves arranged in transverse or quasi-transverse direction; in this way, the strong flexure upon entrance into and departure from contact is facilitated by the opening of the notches, and thus the energy necessary for the rolling and accordingly the energy consumed by the tire are reduced.
If one considers what happens during the rolling of a tire comprising an unnotched tread, it is noted that, in the zone of contact, the tread is subjected to the action of pressure forces perpendicular to the surface of contact with the ground, which forces counterbalance the load borne by the tire, to the action of shear forces tangent to the surface of contact and directed both longitudinally and transversely, and to internal compression forces resulting from the variation in curvature of said tread. The tangential forces are the direct consequence of the contact between the ground and the rolling surface of the tread of the tire and the existence of rubbing between said surfaces. Under the effect of the compressive forces resulting from the flexure of the tread and taking into account the existence of the forces tangent to the rolling surface, there is produced a deformation of the tread in the direction of the thickness which is equivalent to a thickening of said tread. This deformation in the direction of the thickness of the tread is, in part only, limited by the contact forces acting perpendicular to the rolling surface in the zone of contact and the inflation pressure forces acting within the tire.
On the other hand, in the case of a tread which is extensively notched by numerous cuts so as to produce continuous ribs and/or blocks of rubber, the tread elements thus formed are subjected to the actions of the contact pressures and the inflation pressures and there results from this a compressive deformation along their thickness, which leads to the dissipating of a certain amount of energy upon each cycle, that is to say, upon each passage into contact.